This invention relates to ultrasonic position sensing and, more particularly, to an ultrasonic system for detecting the relative position of a moveable device, such as the piston of a fluid cylinder, relative to structure containing such device.
It has been proposed previously, as in Tellerman U.S. Pat. No. 3,898,555 and Biller et al U.S. Pat. No. 4,140,044, to employ a sonic delay line for continually monitoring the displacement of a hydraulic actuator piston. The delay line is configured, in such prior apparatus, to extend along the length of travel along the piston. An annular magnet is moved along the line by the piston, thereby providing a point of reaction for causing wave propogation along the delay line. The relative complexity of such an arrangement makes it relatively impractical for most applications.
There exist a wide variety of kinds of apparatus in which a device is moveable within a fluid-filled structure or vessel. Thus, there are many configurations of fluid actuators wherein a piston or equivalent device is moveable within the cylinder, such as hydraulic or pneumatic cylinders wherein fluid displacement causes, or is the result of, piston movement. The use of a transducer for ultrasonically monitoring the position of such piston or other device is hampered by errors resulting from the velocity of sound which, being dependent upon the medium in which it travels, is subject to variation resulting from changes in pressure or temperature of the fluid occupying the space between the transducer and the piston or other device. For example, in air, the velocity of sound may vary from about 330 me.-sec..sup.-1 to over twice that value over a temperature range of 0.degree.-1000.degree. C. Other fluids exhibit changes in the velocity of sound upon change in parameters characteristic of the fluid. Thus, in a hydraulic cylinder subject to variations in hydraulic pressure of several thousand p.s.i., the simple placement of a transducer at one end of the cylinder will not enable accurate determination of the position of the piston within the cylinder because of wide temperature and pressure excursions.
Limit switches have been used to detect the position of a piston within a hydraulic cylinder, such as upon reaching one end of the cylinder. Other mechanically-actuated devices have similarly been used, such as for controlling a pneumatic valve in response to movement of a piston within a pneumatic cylinder to a predetermined location. Such switches and other mechanical devices are prone to failure, are more expensive or cumbersome to use than desired, and are impractical for continually monitoring the position of, for example, the piston within a fluid-filled cylinder. Accordingly, there has been resort to such cumbersome expedients as the use of shaft position encoders and the like where it is desired to determine the position of a piston at any given moment, as in Novak U.S. Pat. No. 4,121,504 teaching the use of a rotary transducer associated with a shaft turned by movement of a piston within a hydraulic cylinder.
There are many applications for a system which can accurately determine, on a continuous basis, the relative position of a moveable device within a fluid-filled structure.
It is an object of the present invention to provide a system for accurately ultrasonically detecting the relative position of a moveable device within a fluidfilled structure containing the device.
It is another object of the invention to provide such a system which is particularly useful for accurately and continually monitoring the relative position of a piston within a cylinder where the cylinder contains a fluid or other medium which, by reason of changes in parameters characteristic of the fluid or, for other reasons, would produce error in position ultrasonic detection resulting from changes in the velocity of sound in the fluid.
It is a further object of the invention to provide such an ultrasonic position detecting system which can readily be employed with a wide variety of fluid-filled cylinders, such as hydraulic actuators, pneumatic cylinders, and myriad devices wherein a piston or other moveable device is moveable within structure containing a fluid through which ultrasonic energy must travel for position detection purposes.
Additionally, it is an object of the present invention to provide such an ultrasonic detection system which electronically can develop position or control signals which are a function of the instantaneous position of a moveable device within a fluid-filled structure.
Among other objects of the invention may be noted the provision of an ultrasonic system for accurately detecting pressure or temperature; which can be used for monitoring both position and parameters such as temperature and pressure; and which is useful for controlling the position of a moveable device, by continually, accurately monitoring its position, as a complex function of time.
Briefly, an ultrasonic system of the invention is useful for detecting the position of a movable device, such as a piston, within a fluid-filled vessel, such as a hydraulic or other fluid cylinder, containing the device, relative to structure of the vessel. An ultrasonic transducer is positioned, as at one end of the cylinder, for transmitting ultrasonic signals from such location to the device, where the signals are reflected, and for picking up the ultrasonic signals after they are reflected by the device. Circuitry is interconnected with the transducer and responsive to the time interval between the transmitted and picked-up signals for generating an electrical output signal which is a function of the position of the device within the vessel. Thus, in the case of a fluid cylinder, wherein a piston may be driven by fluid pressure over a range of possible positions within the cylinder, the function of the position of the piston along its course of travel at any given time is continually monitored. Further, the system includes correcting means for causing the output signal to be corrected as a function of at least ond parameter characteristic of the fluid which is subject to causing error of the output signal. Such parameter may be the fluid pressure or temperature, changes of which are otherwise prone to cause error in the position output signal because of variations in the velocity of sound transmitted through the fluid.
An ultrasonic system of the invention may also include circuity for comparing the position of the device with the desired limits for other data and for providing control signals as a function of such comparison, as for control purposes or to signal the coincidence of the device with a desired position. An ultrasonic system of the invention may also be utilized for pressure or temperature detection as in response to a pressure-reduced change in position of a moveable device such as a diaphragm, aneroid, bellows or piston.
Other objects and features will be in part apparent and in part pointed out hereinbelow.